1. Field
The present invention relates generally to printing, and particularly, to printing on cylindrical objects, such as cans, and substantially cylindrical objects, such as bottles via simultaneous axial and circumferential nozzle deposition interlacing in such a manner as to increase print resolution and commercial printing speeds.
2. Description of the Problem and Related Art
Current methods of printing indicia on cylindrical objects, such as cans or bottles, via digital printing with commercial inkjet printheads is known in the art. While these methods employ systems traditionally designed for flat surface printing, the adaptation to cylindrical printing imposes efficiency issues affecting print speed and quality, especially for multi-color applications. Printhead efficiency being largely a result of maximum printhead firing uptime, is compromised when printing cylindrical or substantially cylindrical objects with color over color printing, as is well known in the art.
Ink jet printing is well-known, and because it can be digitally controlled using a computer, it has the flexibility to allow a user to change designs as desired. Only recently, however, have advances in technology been made to enable true image rendering on non-planar objects. For example, U.S. Pat. No. 7,111,915 entitled, Methods and Apparatus for Image Transfer, issued Sep. 26, 2006, to Martinez, and LaCaze (the inventor herein), and which is incorporated herein fully by reference, describes an ink jet printer for the printing of indicia on non-planar objects such as baseball bats. Multiple bats are held in a horizontal carousel structure and are positioned relative to one to four printheads, each of which is dedicated to one of four colors: cyan, magenta, yellow and black. Each bat is then rotated in relation to a printhead which is computer-controlled to apply ink according to a programmed image file. However, because the printheads by necessity are arranged in series, the time required to complete a multi-color inkjet application increases with the addition of more colors, even though continuous, helical-type printing may be employed individually for each color.
Another example of printheads serially aligned is found in U.S. Pat. No. 8,931,864, entitled, Apparatuses for Printing on Generally Cylindrical Objects and Related Methods, issued Jan. 15, 2015, to LaCaze and which is incorporated fully by reference, describes an inkjet printer for the printing of indicia on generally cylindrical objects. A plurality of stationary digital printheads are arrayed in an arch oriented perpendicularly to a linear path along which the object to be printed is conveyed. An object, such as a can or bottle, is positioned relative to the arch and rotated about the objects long axis as the printheads eject ink. However, the object is incrementally advanced along the linear path i.e., indexed without the printheads jetting ink, which detracts from printhead firing efficiency and overall print speed.
To illustrate the problem, FIG. 1 depicts, an object to be printed 1 in relation to four printheads 2a-2d arrayed in an arch traversing the line of travel for the object which corresponds to the object's long axis. The object 1 is shown outside the start of the nozzle array which marks a plane intersecting the object's line of travel that once breached by the object, nozzles begin depositing ink upon the object's 1 surface. The object is indexed along the line of travel, i.e., axially, and rotated.
FIG. 2 depicts the apparatus from the side where the object 1 has advanced a sufficient distance, such that the object leading end (or the beginning of the intended print area of the object 1) is in line with the end of the nozzle array. As is shown here, it is possible—and in practice usually the case—that the length of the object to be printed 1 exceeds the available print length afforded by the digital printhead(s) 2a-2d in question.
FIG. 2a shows the object to be printed 1 linearly advanced further by a distance equal to the available print length afforded by the digital printhead(s) 2a-2d. The object 1 will continue to advance in steps equal to this same distance until the entire length of the object 1 is printed. Typically, this is repeated as many times as required to attain the desired print resolution, the number of passes depending upon the native resolution of the printheads 2a-2d. There are several problems maximizing the speed and resolution utilizing this state-of-the-art technology. Minimization of the time required to print the object 1 requires, among other criteria, the most efficient use of the printheads 2a-2d. This occurs when the printhead 2a-2d nozzles are firing (versus idle), that is, depositing ink, toner, etc. to the object 1 as is well known in the current art. The time necessary to print the object 1 increases as the printhead 2a-2d nozzle idle time increases. This occurs for each of the printheads 2a-2d when the object to be printed 1 is advancing to arrive at the next printing position, as the printheads 2a-2d do not fire during this movement. Additionally, print quality may suffer because axially indexing of the object 1 to be printed can result in print stitch lines that appear as lines demarking the boundaries between adjacent printed areas. Stitch lines are usually dealt with by blending adjacent printed areas together along the stitch line, but may still be observable and unappealing depending upon the accuracy and repeatability of object 1 positioning.
Another opportunity for printhead idle time with this arrangement is illustrated in FIG. 3. In the practical application of this technology, it is often desirable, and even necessary, to print the desired pattern on the object 1 by applying colors each other in a specific sequence, for example, applying yellow, cyan, magenta and black, specifically in that order. This example illustrates one of the common dictates of process printing, namely printing from “light” to “dark” colors in progression. In FIG. 3 the first digital printhead 2a would therefore print yellow, the second digital printhead 2b cyan, the third digital printhead 2c magenta, and the fourth digital printhead 2d black. Given when printing, the object 1 is rotating, but axially stationary, printhead 2a fires its nozzles first; printhead 2b only fires its nozzles as the print area of the object surface begins to pass beneath it; 2c fires as the print area f begins to pass beneath it, and so on.
Because of the lag between 2a and 2d, the object 1 must complete more than one rotation to complete the desired print while at the same time the object 1 must be axially advanced to account for the difference between its length and the length of the available print area, again resulting in decreased efficiency. Further, there is a period when all printheads 2a-2d are firing, but at the end of print, the process is reversed: the first printhead 2a stops firing while all other printheads 2b-2d are still firing; the second printhead 2b stops while the third printhead 2c and the fourth printhead 2d are still firing; and the third printhead 2c stops while the fourth printhead 2d is still firing. This cumulative lag time at the beginning and ending of the printing indexes has a deleterious effect upon the time it takes to print the object 1. Increasing the desired print resolution to be greater than the native printhead 2a-2d resolution only serves to exacerbate this problem by requiring additional print deposition(s) and indexes.
U.S. Pat. No. 8,926,047 entitled, Apparatuses for Printing on Generally Cylindrical Objects and Related Methods, issued Jan. 6, 2015, by LaCaze et al. (the inventor herein) incorporated herein fully by reference, addresses printhead inefficiency during simultaneous axial and rotational motion by offsetting the printheads in an axial direction relative to the long axis of the object to be printed. However, this creates a problem in that the degree of offset must be different object diameters as well as different print patterns and resolutions, potentially resulting in significant lost production time.